This invention relates to a gain-controlled amplifier, and, more particularly, to a gain-controlled amplifier the gain of which is controlled in accordance with a control voltage externally applied thereto.
FIG. 1 shows a prior art analog multiplier which may be used as a gain-controlled amplifier. The prior art gain-controlled amplifier is comprised of a first pair of differential transistors 1 and 2, a second pair of differential transistors 3 and 4, and a third pair of differential transistors 5 and 6. The transistors 1 and 4 have their collectors connected to a positive power supply terminal through collector load resistors 7 and 8, respectively, and the transistors 2 and 3 have their collectors connected to the collectors of transistors 4 and 1, respectively. The transistors 2 and 3 have their bases commonly connected to circuit ground, and the transistors 1 and 4 have their bases commonly connected to receive a control signal Vc. The transistor 5 has its collector connected to the commonly connected emitters of transistors 1 and 2 and its base connected to receive an input signal Vi to be amplified. On the other hand, the transistor 6 has its collector connected to the commonly connected emitters of transistors 3 and 4 and its base connected to circuit ground. The emitters of transistors 5 and 6 are commonly connected to a negative power supply terminal through a constant current source 9. An output signal Vo of the prior art circuit is obtained by subtracting collector voltage of one of the transistors 1 and 4 from collector voltage of the other of the transistors 1 and 4, and represented as follows: EQU Vo=K.multidot.Vi.multidot.f(Vc) (1)
where K is a proportional constant and f(Vc) is a function of the control voltage Vc. The gain A of the circuit is, therefore, represented by ##EQU1## That is, the gain of the circuit of FIG. 1 varies with the control voltage Vc and is substantially proportional to the magnitude of the control voltage Vc when it is relatively small.
For the purpose of decreasing the distortion factor of the above-mentioned gain control amplifier, the amplifier must be operated as a Class A amplifier. For Class A operation, the emitter current of each transistor in the circuit must be increased. However, the increase in the emitter current of transistor correspondingly increases noises produced within the amplifier circuit. Conversely, when the emitter current is decreased so as to reduce noises, the linear operating range of the amplifier will be decreased. As a result, the distortion factor of the amplifier will be increased. That is, with the gain-controlled amplifier of FIG. 1, it is not easy to reduce both of noises and distortion factor. Further, since, in the gain control amplifier of FIG. 1, the collectors of a pair of transistors 1 and 3 are commonly connected to the collector load resistor 7 and the collectors of another pair of transistors 2 and 4 are commonly connected to the collector load resistor 8, the current flowing through each of the collector load resistors 7 and 8 does not change with the control voltage Vc so that noise output voltage does not change as well. As a result, when the gain of the amplifier circuit is set small by the control voltage Vc the signal-to-noise ratio of the amplifier circuit will be deteriorated.